Liquid scintillation counters are commonly used to measure the count rate or activity of samples containing low energy beta particles or corresponding particles emitting radionuclides such as tritium, iodine-125, carbon-14, sulphur-35, calcium-40 and chromium-51.
The range of the low energy beta particles in the sample is in generally few tens of micrometers at the most. As a consequence, the sample to be measured has to be placed in direct contact with a scintillation medium by dissolving or suspending the sample within the liquid scintillation medium in a container so that the emitted beta particles can interact with the molecules of the liquid scintillation medium, which comprises a solvent or solvents and a solute or solutes present in a few percent by weight of the solution. In this interaction process most of the kinetic energy of the interacted beta particle is absorbed by the solvent and then transferred to the solute which emits scintillation photons, whose amount is proportional to the energy of the interacted beta particle. These scintillation photons are detected by two, in coincidence operating, photomultiplier tubes producing electric pulses. The sum pulse height is proportional to the energy of the interacted beta particle.
Traditional liquid scintillation counters are provided with one detector and they are designed to measure samples in 7 ml or 20 ml glass or plastic vials.
In addition to the above mentioned conventional liquid scintillation counters three special purpose liquid scintillation counters exist, which counts samples directly from filter mats or multi-well sample plates.
First special purpose liquid scintillation counter is shown in U.S. Pat. No. 4,298,796, which apparatus counts liquid scintillation or corresponding samples directly from support layer, which is disposed within a container which contains the liquid or gel scintillant. The apparatus has one or several detectors in order to count one or several samples at a time.
Second special purpose liquid scintallation counter is shown in U.S. Pat. No. 5,061,853, which apparatus counts liquid scintillation or corresponding samples directly from sample plates which comprises several separate sample wells or vials. The apparatus has one or several detectors in order to count one or several samples at a time. The sample plate is placed in the counting position or before counting position manually or automatically on a rigid plate holder made of photon attenuating material and having holes for the wells of the sample plate. The walls of the holes are reflecting or scattering in order to guide the photons from the liquid scintillation sample to the detectors, built of two photomultiplier tubes operating in coincidence and situated on the opposite sides of the holes of the plate holder. The wells of the sample plate can be closed by an adhesive transparent tape. The apparatus can be used also for counting gamma radiation emitting samples if the holes of the sample plate are surrounded by gamma radiation sensitive detectors.
Third special purpose scintillation counting system for in-situ measurement of radioactive samples in a multiple-well plate is presented under European Patent Publication Number 0425767 A1. This apparatus is provided with multiple photomultiplier tubes positioned adjacent to the sample wells containing the scintillator for simultaneously measuring the radioactivity of multiple samples with only a single photomultiplier tube sensing the scintillations from each well and converting the sensed scintillations into corresponding electrical pulses. The electrical pulses from each photomultiplier tube are processed to discriminate between pulses attributable to sample events within the wells and pulses attributable to non-sample events such as photomultiplier tube noise. The discrimination is effected by determining whether a selected number of electrical pulses occurs with a prescribed time interval, the occurrence of the selected number of pulses within the prescribed time interval signifying a sample event. Only the electrical pulses attributable to sample events are supplied to a pulse analyzer.
The multi-well sample plates have typically eight rows of wells, whose diameter is 7-8 mm arranged in twelve columns with 9 millimeters distance between the center points of the wells. The typical volumes of sample wells of such 96-well sample plates are 200-400 microliters depending on the height of the plate.
When measuring sample activities with liquid scintillation counters, the basic problem is the reduction of the counting efficiency due to the quenching of the sample, which can be classifield in two main types: the chemical quench and the color quench. The chemical quench is a phenomenom, where the solution formed by the sample and the scintillation medium contains some impurities, which reduce the efficiency of the counting system to detect the emitted beta particles by absorbing them. The color quench is a phenomenom, where the solution formed by the sample and the scintillation medium contains some impurities, which absorb produced scintillation phtotons. The consequence of this is also the reduction of the counting efficiency.
It is known in the liquid scintillation counting that the reduction of the counting efficiency due to the quenching of the sample can be corrected by a means of a quench curve which describes the relationship between the counting efficiency and the amount of the quench of the sample. The quench curve is obtained by measuring a set of standards, which have same activity but different amount of quench. Standards are prepared for example in the following way:
1) 10 ml scintillation liquid is pipetted into 20 ml sample vials. Normally 5- 10 vials are used. PA1 2) A constant amount of activity of used isotope is added into each vials. The vials are shaken well and the solution is often allowed to stand overnight. PA1 3) The standard solution samples in vials are quenched by adding an amount of quencher, for example carbontetrachloride (CCl-4) which is a chemical quencher, into the vials, a different volume into each vial. Instead of a chemical quencher a color quencher, for example appropriate color dye, can be used. Carbontetrachloride is added for example as follows: PA1 If samples to be analyzed are deposited and counted directly from sample plates which comprises several separate sample wells or vials, then preparation of the standards continues as follows: PA1 4) From each of the standard vials for example 200 microliters of quench standard solution is pipetted into the appropriate wells on the sample plate.
______________________________________ Vial No CCl-4 (microliters) ______________________________________ 1 0 2 5 3 10 4 15 5 30 6 50 7 70 8 90 9 120 10 150 ______________________________________
In U.S. Pat. No. 4,933,554 (Lehtinen, Yrjonen, Ostrup) is shown a method of producing a carrier for a plurality of radioactive sample to be monitored in a liquid scintillation counter. According to this method sample plate is sealed with two photon permeable adhesive foils and at least one of the foils can be colored in order to simulate quenching. Due to the expensiveness of producing colored foils with exactly variable optical densities and spectra, the present invention shows a more reliable and simplier method, where standardization samples are quenched by covering them with photon permeable foils, which are provided with small separate black dots arranged in matrix form. Those scintillation photons produced by sample which hit these black dots are absorbed. Thus quenching of the sample depends on the relative total area of the black dots on the foil.